In recent years, biodegradable polymers which are decomposed in the natural environment have been attracting attention and investigated worldwide for global environmental protection. Polyhydroxy butyrate, polycaprolactone, aliphatic polyesters and polylactic acid are known as the biodegradable polymers. Since polylactic acid is obtained from lactic acid obtained from a raw material derived from living organisms and a derivative thereof, it is an environment-friendly polymer material having high bio safety. Therefore, use of polylactic acid as a general-purpose polymer is now under study, and its application to medical goods such as surgical suture threads, sustained-release capsules and reinforcement materials for broken bones as a stretched film, fiber or injection molded article is being investigated.
However, polylactic acid has a low crystallization rate and its use as a molded article obtained by crystallization is limited. For instance, it takes long to injection mold polylactic acid because its molded product must be crystallized, and a heat treatment may be required after molding.
Further, polylactic acid has low wet heat stability that it is easily decomposed by water at a high temperature.
To improve the crystallization rate of polylactic acid, a method in which a crystallization nucleating agent for promoting crystallization is added has been studied, and the crystallization rate has been improved to a certain extent. For example, it is proposed to use a phosphoric acid ester metal salt in combination with polylactic acid and it is disclosed that the salt is effective for the crystallization of the resin (patent document 1). However, when the phosphoric acid ester metal salt is used, a new problem occurs that wet heat stability becomes worse. Further, the addition of the phosphoric acid ester metal salt may greatly reduce the molecular weight.
As an alternative method, it is proposed to add an inorganic filler such as a glass fiber. However, when the inorganic filler is used, the specific gravity of the obtained molded product increases. Further, when it is burnt or scrapped, a large amount of the residue is produced, thereby impairing one of the features of environment-friendly polylactic acid. It is also proposed to add a natural organic filler such as wood powder or kenaf to polylactic acid (patent document 2). However, a cellulose-based filler has low heat stability at the molding temperature of polylactic acid by its nature and deteriorates and decomposes in a molding machine at the time of molding. Therefore, it cannot be used to manufacture a high-quality molded article stably.
As for the improvement of the wet heat stability of polylactic acid, various stabilizers such as hindered phenols are proposed but not satisfactory in the improvement of the wet heat stability of polylactic acid.
It is also proposed to improve the wet heat stability by modifying the terminal group of the molecule of polylactic acid which involves in wet heat decomposition. For example, an epoxy compound (patent document 3), carbodiimide compound (patent document 4), and oxazoline, oxazine and aziridine compounds (patent document 5) are proposed. The wet heat stability of polylactic acid is improved to a certain extent by using the above terminal group modifier. However, the terminal group modifier plasticizes a resin and deteriorates its crystallinity. The additive itself has low heat resistance, thereby deteriorating the color of the resin significantly at the time of molding and reducing its value as a commercial product.
Meanwhile, it is known that stereocomplex polylactic acid is formed by mixing together poly(L-lactic acid) and poly(D-lactic acid) in a solution or molten state (patent document 6 and non-patent document 1). This stereocomplex polylactic acid has a melting point of 200 to 230° C. which is higher than those of poly(L-lactic acid) and poly(D-lactic acid) and high crystallinity.
However, the stereocomplex polylactic acid does not show the single phase of stereocomplex polylactic acid and is a mixed-phase composition having the phases of poly(L-lactic acid) and poly(D-lactic acid) (may be referred to as “homo phase” hereinafter) and the phase of the stereocomplex polylactic acid (may be referred to as “complex phase” hereinafter). According to the DSC measurement of the stereocomplex polylactic acid, a low melting point crystal melting peak at a peak temperature of 190° C. or lower corresponding to the melting peak of a homo-phase crystal and a high melting point crystal melting peak at a peak temperature of 190° C. or higher corresponding to the melting peak of a complex-phase crystal are observed.
Since the stereocomplex polylactic acid has the complex phase and the homo phase, it has a defect that the heat resistance of the stereocomplex polylactic acid is hardly obtained fully. To obtain the heat resistance of the stereocomplex polylactic acid, the stereo crystal rate (S) defined by the following equation (a) must be not less than 80%.S=[ΔHmsc/(ΔHmh+ΔHmsc)]×100  (a)
In the above equation (a), Hmh is the crystal melting heat of a crystal melting peak at a temperature lower than 190° C. corresponding to the melting of a homo-phase crystal observed in the DSC measurement, and ΔHmsc is the crystal melting heat of a crystal melting peak at a temperature of 190° C. or higher corresponding to the melting of a complex-phase crystal observed in the DSC measurement.
To cope with this problem, the use of a crystallization nucleating agent such as a phosphoric acid ester metal salt is proposed, and a heat-resistant composition containing no homo-phase crystal but containing only a complex-phase crystal having a crystal melting point of 209° C. and a molded article thereof are proposed (patent document 1).
Even the stereocomplex polylactic acid containing a crystallization nucleating agent such as a phosphoric acid ester metal salt does not still solve a molecular weight reduction problem which occurs in the above-described poly(L-lactic acid) and poly(D-lactic acid) and has a defect that it is difficult to retain its physical properties due to a great reduction in weight average molecular weight. The stereocomplex polylactic acid is classified into a group of aliphatic polyesters and has a defect that it is readily hydrolyzed by moisture as a feature of the aliphatic polyesters like poly(L-lactic acid) and poly(D-lactic acid). Therefore, the improvement of the wet heat resistance of the stereocomplex polylactic acid is desired.    (Patent Document 1) JP-A 2003-192884    (Patent Document 2) JP-A 2005-2174    (Patent Document 3) JP-A 2002-115121    (Patent Document 4) JP-A 11-80522    (Patent Document 5) JP-A 2002-30208    (Patent Document 6) JP-A 63-241024    (Non-patent Document 1) Macromolecules, 24, 5651 (1991)